A team of civil rights and prisoner rights groups are intensifying a campaign to thwart a statewide crime reform bill, saying the state should not rush into a decision on legislation that could send hundreds of defendants to prison without the possibility of parole.

The groups, in letters to legislative leaders, have particularly taken issue with the Three Strikes component of the package, which would mandate life in prison without parole for people convicted three times of certain crimes. It would also require a habitual offender to serve a maximum prison sentence with reduced eligibility for parole on other offenses.

A legislative conference committee is slated to discuss the crime and sentencing bill, which would also include changes to drug sentences and the way other sentences are handed out, as soon as Thursday. The House and Senate passed competing versions of the bill last year.

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The team of civil rights groups, including the American Civil Liberties Union, the Black Ministerial Alliance, and Prisoners’ Legal Services, is sending a letter to the conference committee, Governor Deval Patrick, and Supreme Judicial Court Chief Justice Roderick L. Ireland. The letters call for officials to table the proposal and have an outside national research group study crime trends in Massachusetts and recommend a proper reform package.

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The letter states that the Pew Center on the States had conducted a Public Safety Performance Project in other states, and that the reports helped lead to agreeable, data-driven crime reforms that reduced recidivism and saved tax dollars.

Meanwhile, the Charles Hamilton Houston Institute for Race and Justice at Harvard Law School is set to release a study Thursday that found the crime package, as proposed by both the House and Senate, could make hundreds of low-level offenders eligible for lengthy prison sentences, which would cost the state tens of millions of dollars a year.

The study said the package would also overwhelm prisons that are already beyond capacity and would only strengthen laws that disproportionately affect minorities.

Supporters of the bill have said the legislation is necessary for public safety, to keep the state’s most dangerous criminals behind bars.

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“It captures those who, quite frankly, have no place in society,” said state Representative David P. Linsky.

The movement behind the law was based on the December 2010 fatal shooting of Woburn police officer John Maguire during a robbery of a department store. He was killed in a shoot-out with Dominic Cinelli, a career criminal who had been sentenced to multiple life sentences, but was out on parole.

The killing triggered sweeping changes to the state Parole Board, as well as an appetite for a comprehensive crime reform package, with proponents arguing that Cinelli, who was also killed in the shoot-out, should never have been released from prison.

State Senator Sonia Chang-Diaz of Boston said in an interview Wednesday that she will wait to see whether the conference committee settles on a final package for the Senate to decide on, but that she wants to see a package different from what has been proposed. The recommendations have not gone far enough to scale back mandatory sentences for non-violent drug offenses, which disproportionately affect blacks and Latinos, she said. She also said the Three Strikes law goes too far in punishing defendants convicted of certain crimes.

She acknowledged the passion for a reform package after the death of Maguire, but said the state should wait for a comprehensive study, rather than a crime bill based on “emotionally charged” instances.

Milton J. Valencia can be reached at mvalencia@globe.com. Follow him on Twitter @miltonvalencia.